European Patent EP 1,313,112 [U.S. Pat. No. 6,709,496] discloses an air dehumidifier of the above-described type in which an electric heater is provided to heat up and thereby dry the regenerable absorption agent. It is possible in this way to redry the absorption agent when saturated and thereby to make it ready for further liquid uptake. The electric heater in this case is actuated by a moisture sensor mounted in the air dehumidifier when a threshold value is exceeded.
German Patent DE 103 57 085 [US 2007/0199443] further discloses a method of dehumidifying with a heatable air dehumidifier of this type. The electric heater for heating up and drying the regenerable granules is only actuated here when the moisture sensor signals that a moisture limit has been exceeded and also when there is no air flowing into the oil expansion tank. In other words in this method, actuation of the heater is permitted only when no air inflow is present or when air is escaping from the oil expansion tank, but never when air is entering the oil expansion tank.
These states are detected in the prior-art method by sensing air flow and optionally its flow direction or also by determining the pressure difference between the dehumidifier housing and the oil expansion tank. Air flow sensors or even relative pressure sensors are used for this purpose.
It has become evident in practice that air flow through the actual oil expansion tank is relatively low. A typical value in practice for power transformers is 2.5 l/min and for tap changers 0.5 l/min. With these low air throughputs, the air-flow sensors used have to be highly accurate; such sensors are costly.